A single layer of atomically sp2 bonded carbon atoms, called graphene is traditionally manufactured using methods such as micro mechanical exfoliation, chemical vapor deposition and laser ablation of graphite. These methods are generally expensive, need capital equipment, or need usage of large number of steps in producing graphene or graphene oxide.
A catalytic chemical liquid deposition (CCLD) process to grow carbon nanotubes (CNT) and carbon nanowires (CNW) and graphene on to a silicon substrate is known in the literature. The substrate was first prepared and coated with a Fe film using electron beam evaporation. CCLD using various growth parameters included changing the voltage, organic liquid, organic liquid-to-water ratio, reaction time, and sweep gas flow to achieve CNT, CNW and graphene growth. All tests were run under atmospheric pressure and consistent temperature (298 K). The surface composition was characterized using a SEM. Energy dispersion X-ray (EDX) and resonance Raman spectra were collected. The process was able to grow CNTs, CNWs, and graphene at a high rate as well as illustrate the growth process of these structures. See U.S. patent application Ser. No. 2003/0217928 to Yuehe Lin, Liang Liang, and Jun Liu.
It is known to use an ambient pressure chemical vapor deposition (APCVD) process to grow carbon nanotubes (CNT) and graphene onto copper foil. Methane gas was introduced on 1-5 nm Fe catalysts that were deposited onto the surface using electron beam evaporation. The substrate was then heated to 1023 K. C2H4 was introduced to promote CNT growth. SEM imaging was used to characterize the surface morphology. CNT and graphene were successfully deposited onto a copper substrate using APCVD. Another publication also used an APCVD process to successfully grow high quality graphene on copper substrates, albeit without catalysts applied to the substrate.
It is known to utilize a hot filament chemical vapor deposition (HFCVD) technique to deposit bamboo-like carbon nanotubes (BCNTs) onto a copper substrate. A tungsten filament was carburized in the HFCVD chamber before the deposition process. The copper substrate was polished with various grit sizes until finally being polished with <1 micron diamond powder. A gas mixture of 2.0% CH4, 98% H2 with 500 ppm of H2S at a pressure of 20 torr was used and the filament temperature was maintained at ˜2773 K. The substrate temperature and deposition time were altered to investigate different BCNT growth patterns. Surface characterization was accomplished using SEM and Raman spectroscopy. The BCNTs were deposited onto a copper substrate without any catalyst and found that as the substrate temperature increases, approaching the copper melting point, the BCNT growth patterns changed from a microscystalline diamond to a dense entangled network of CNTs.
It is known to use a Low Pressure Chemical Vapor Deposition (LPCVD) set-up to grow graphene layers on copper foil. First the copper foil substrate was annealed and placed in the LPCVD chamber and a gas mixture of CH4, Ar and H2 was introduced into the chamber. Once deposition was complete the chamber temperature was quickly cooled to approximately 673 K. The purity of the copper film was found to be a key factor in controlling the graphene synthesis as well as the minimum partial pressure of the hydrocarbon.
Other processes have also employed chemical vapor deposition (CVD) to successfully grow graphene/CNT on to copper substrates. A CVD process was used to deposit graphene-CNT hybrids onto copper foil. The copper foil was coated with silicon nanoparticles (Si NPs) and placed in quartz tube furnace at 1073 or 1173 K. Then ethanol vapor was introduced to the tube to promote graphene-CNT hybrids. The surfaces were then characterized by using a scanning electron microscope (SEM) and transmission electron microscope (TEM). The surface characterization showed that the graphene-CNT hybrids sprouted from the Si NPs. This method provides a way to obtain the desired CNT properties by altering growth conditions and Si NP size, density and arrangement. Another method successfully tested methanol, ethanol and 1-propanol as gas mixtures during CVD, potentially making the fabrication of graphene, more accessible, cheaper and safer.
Others developed an electrochemical exfoliation process for the deposition of graphene oxide (GO) onto a copper foil surface. The GO was washed and placed in an electrolytic bath of 250 g/L copper sulphate. In addition H2SO4 was added to maintain pH, and PAA5000, a surfactant, was added to prevent agglomeration. The optimum content dispersion was found to be 0.5 gm/L. The electrodeposition system contained an electrolytic Cu (99.99%) anode and a titanium cathode. Note that the titanium cathode first had a 2 μm Cu film applied before deposition to act as a seed layer. The document characterized the surface morphology, studied the distribution of graphene in the composite foil, and examined the product using a Field Emission Scanning Electron Microscope, Focused Ion Beam, and Transmission Electron Microscope. The methodology achieved superior mechanical properties as a result of the composite nature of the product, while at the same time maintaining the electrical conductivity of pure Cu.
There exists a significant body of research pertaining to the study of depositing CNTs or graphene onto different substrates, and as it more relates to, copper. CVD appears to be the most common method of deposition and has resulted in successful deposits. However, this method, as well as other variants (HFCVD, APCVD, plasma enhanced chemical vapor deposition (PECVD)), requires specific equipment, materials, and some also involve relatively complex preparation and procedure.
In patents granted on the electrochemical production of graphene, electrolytes have been added into the medium to reduce the resistance of the medium. Addition of an electrolyte into a medium is generally known to increase the conductivity of the medium. Since conductivity is reciprocal of resistance, addition of an electrolyte reduces the electrical resistance of the medium. Thus, the medium property is altered. In addition in some patents the electrolyte added undergoes oxidation at the anode or reduction at the cathode. These processes initiate the formation of graphene, however require an electrolyte added to the electrolytic bath.
The CVD methods and the existing electrolytic methods require multiple steps for producing graphene or graphene oxide. The CVD methods require the use of metal catalyst for the decomposition of the starting material. The end product of these methods is graphene/graphene oxide containing the metal catalyst. The catalyst will have to be removed for obtaining pure graphene/graphene oxide. This removal involves either chemical treatment to complex the metal or removal by physical or chemical methods. The electrolytic methods produce impure graphene/graphene oxide in a bath containing surfactants or electrolytes that need to be removed by physico-chemical methods. Both the methods involve multiple step processes.
Electrolytes are substances that dissolve in water to produce ions which carry the current in electrolysis. The electrolytes are generally composed of a metal and a non-metal. When dissolved in water, the metal part of the electrolyte forms its ion (cation) and the non-metal part of the electrolyte forms its ion (anion). The negatively charged anion will have a tendency to get oxidized at the anode and has been used in known electrochemical processes utilizing an electrolyte for cleaving the graphite to produce graphene. However, in such cases, the presence of electrolytes in the bath results in byproducts which then have to be removed from the solution. In a large number of cases electrolytes containing ions, such as iodide ion, bromide ion, chloride ion and the like, the oxidation of these ions precedes the oxidation of water and hence will control the extent of the graphene cleavage. Thus, the art lacks an efficient single step electrochemical process for the production of pure graphene/graphene oxide.